I'm working on a friends 95 pickup. He said he was driving it and went to shift to the next gear and the shifter was locked up, as in it doesn't move anywhere anymore. I confirmed this by trying to get it into a gear myself. The thing is locked up. Any ideas what this could be?

Will it shift through the gears when the truck is off or is it stuck in fifth still? If it is stuck in fifth when the truck is off, you're probably looking at internal transmission problems, at least from my experience. If you can shift through the gears when it's off, you're probably looking at a slave cylinder or clutch. If it's the transmission I believe it is, you should be able to climb beneath of it and see a rubber boot. Remove the boot and have your buddy work the clutch. If you see movement from the slave cylinder when he hits the pedal and the clutch looks okay, you probably have bigger problems. I'm no expert, but I've run into this before. I have dealt with quite a few of those transmission issues.